This invention relates to a fuel control system for a gas turbine engine and more particularly to a fuel control system for controlling fuel flow.
Typically, gas turbine engines use a positive displacement pump to pressurize fuel prior to metering and delivery. Positive displacement pumps provide fuel flow as a function of pump speed. Fluctuation in fuel pressure from the pump must be accommodated to properly and accurately control fuel flow to the engine. Fuel flow from the pump is delivered to a metering valve that controls a flow area to provide a desired fuel flow rate. Different pressures require different flow areas to provide the same fuel flow; therefore fluctuations in pressure across the metering valve must be accommodated.
Typically, a pressure-regulating valve is used to maintain constant pressure across the metering valve. The metering valve position is set by a separate valve or solenoid to provide the proper flow area and the pressure regulating valve maintains the proper pressure differential across the metering valve. In such systems fuel pressure is controlled separate of the flow area of the metering valve. The system operates by first setting the flow area and then setting the pressure in the system to a level relative to the flow area to provide a desired fuel flow rate. A change in the desired fuel flow rate requires an adjustment of the flow area.
Typically, an electronic engine control (EEC) regulates the valves in the fuel delivery system. Each valve controlled by the EEC adds to the complexity and weight of the system. Further, the more valves controlled by the EEC, the less reliable and consistent the control of fuel flow through the system.
For these reasons it is desirable to develop a simplified fuel delivery system that minimizes the number of valves and electronic devices, and maintains accurate and reliable control of fuel flow.
The invention is a fuel delivery system for controlling the flow of fuel to an engine assembly by controlling the fuel pressure of the system relative to an applied current.
The fuel delivery system of the subject invention includes a variable orifice valve acting in conjunction with fixed nozzles of the engine to control the flow of fuel to an engine in proportion to pressure within the system. The variable orifice valve varies a flow area to open proportionally in response to flow through the system. The variable flow area in concert with the fixed area of the engine nozzles creates a substantially linear relationship between system pressure and fuel flow to the engine. A positive displacement pump supplies fuel to the system. The fuel from the pump enters the system at a flow rate and pressure higher than required by the engine. The pressure in the system is set to obtain the desired fuel flow by bypassing a portion of fuel flow back to the pump.
A pressure-regulating valve accommodates fluctuations in pressure by proportionally opening and closing the flow of fuel through the bypass. A pressure setting valve controls the pressure-regulating valve by controlling the flow of fuel through a second bypass. An Electronic Engine Control (EEC) controls the amount of applied current to the pressure setting valve. The pressure setting valve opens the second bypass to fuel flow in proportion to an applied current such that the pressure is proportional to the current, and fuel flow is proportional to the pressure.
In this manner, the flow of fuel corresponds to the level of applied current to the pressure setting valve. The variable orifice valve opens to provide a flow area in proportion to the fuel flow such that the relationship of fuel pressure to fuel flow is substantially linear. Further, controlling current to the fuel setting valve controls pressure within the system which in turn controls fuel flow.
The system of the subject invention requires only the regulation of system pressure to control the fuel flow thereby providing a simplified, and cost effective device for controlling the flow of fuel to an engine accurately and reliably.